When a voltage is applied to an organic electroluminescence device (hereinafter, occasionally referred to as an “organic EL device”), holes are injected from an anode into an emitting layer and electrons are injected from a cathode into the emitting layer. The injected electrons and holes are recombined in an emitting layer to form excitons. Here, according to the electron spin statistics theory, singlet excitons and triplet excitons are generated at a ratio of 25%:75%.
A fluorescent organic EL device, which employs emission caused by singlet excitons, has been applied to a full-color display of a mobile phone, TV and the like. A study for further improving a device performance of the fluorescent organic EL device has been conducted (e.g., Document 1 (JP-A-2008-255095), Document 2 (JP-A-2008-285450), Document 3 (International Publication No. WO2008/111540), Document 4 (International Publication No. WO2008/111543), and Document 5 (JP-A-2010-263065)).
Moreover, an organic EL device employing delayed fluorescence has been proposed and studied. For instance, a thermally activated delayed fluorescence (TADF) mechanism has been studied. The TADF mechanism uses such a phenomenon that inverse intersystem crossing from triplet excitons to singlet excitons thermally occurs when a material having a small energy difference (ΔST) between singlet energy level and triplet energy level is used. As for thermally activated delayed fluorescence, refer to, for instance, ADACHI, Chihaya, ed. (Mar. 22, 2012), “Yuki Hando-tai no Debaisu Bussei (Device Physics of Organic Semiconductors)”, Kodansha, pp. 261-262. An organic EL device using the TADF mechanism is disclosed in, for instance, Document 6 (H. Nakanotani, Nat. Commun., 5, 4016, 2014).